U.S. Pat. No. 5,259,184 describes a configuration for a dual mode, single stage multi-burner combustor designed to achieve low NOx emissions in a pre-mixed operating mode. The invention describes an annular arrangement of fuel nozzles and pre-mixing tubes about the longitudinal axis of the combustor.
Commonly owned, U.S. Pat. Nos. 5,551,228 issued Sep. 3, 1996 and 5,491,970 issued Feb. 20, 1996, both of which are incorporated herein by reference, describe methods of fuel staging in a multi-burner combustor to improve emissions, dynamics and stability. The '970 patent describes a method of fuel staging for a five burner combustor in which the nozzle in one of the five annularly arranged burners is fueled independently of the nozzles in the other four burners which are fed from a common manifold. The fuel from all of the nozzles is injected from radial pegs downstream of a swirler on the nozzle. This first type of staging varies the percent of fuel between the one and the four nozzles in order to improve dynamics and stability of the combustor during the transfer from diffusion to pre-mixed mode and in full pre-mixed mode.
The '228 patent describes the addition of a third pre-mix fuel manifold that supplies fuel equally to all five annularly arranged burners, but which introduces the fuel from radial fuel pegs located upstream of the nozzle swirlers. The second type of staging varies the percent of fuel injected on the upstream versus the downstream side of the swirler. This type of fuel staging is used to reduce combustion dynamics but does so without incurring increases in NOx emissions associated with staging between the one and the four nozzles.
Various problems have been experienced with dual mode, single stage, multi-burner combustors. For example,
(1) During turn-down in pre-mixed operation of multi-burner combustors, the reduction in fuel/air ratio below a critical point causes a sharp increase in carbon monoxide (CO) and unburned hydrocarbon (UHC) emissions and also decreases the stability of the pre-mix flame, ultimately resulting in blow-out. PA1 (2) During low load diffusion operation, multi-burner combustors tend to generate very high CO and UHC emissions. PA1 (3) Optimization of the combustor for pre-mixed operation also has a tendency to reduce diffusion mode flame stability during very low load operation, and can make the combustor prone to blow-out near full speed, no load. PA1 (4) The design of multi-burner combustors for low NOx emissions requires that a high percentage of the combustor air flows through the head end in order to achieve lean fuel-air mixtures. The large air flow results in a high air velocity and large pressure drop across the swirlers in the burner tubes. The high pressure drop impacts gas turbine performance by reducing both efficiency and output. PA1 a) at ignition, supplying the center nozzle with fuel from the center nozzle pre-mix fuel manifold and the two nozzles in line with the cross-fire tubes with fuel from one of the pre-mix fuel manifolds; PA1 b) after detecting cross-fire with the flame detectors, turning off fuel to the two outer fuel nozzles, and supplying all fuel to the center nozzle. Acceleration to full speed, no load (FSNL) and loading to part load occurs by increasing fuel flow to center nozzle; PA1 c) at some part load, begin transferring some pre-mix fuel away from the center nozzle and staging it to the outer fuel nozzles in the following sequence as total fuel flow is increased: 1) center nozzle+two cross-fire nozzles, 2) center nozzle+non-cross-fire nozzles, 3) all outer fuel nozzles in annular array (no center nozzle), and 4) center nozzle+all outer fuel nozzles. PA1 a) at start-up, supplying at least some of the fuel nozzles in the annular array with diffusion fuel from the diffusion fuel manifold; PA1 b) at part speed, supplying pre-mix fuel from at least one pre-mix manifold to one of the nozzles in the annular array; PA1 c) at full speed, part load, transferring those of the fuel nozzles in the annular array supplied with diffusion fuel in step a) to pre-mix fuel; PA1 d) as load is increased, initiating pre-mix fuel supply to the center nozzle without adding to the supply of pre-mix fuel to the fuel nozzles in the annular array; and then PA1 e) supplying additional pre-mix fuel to all of the fuel nozzles in the annular array and the center nozzle to thereby uniformly increase the fuel/air ratio as the turbine load increases.
This problem occurs because the combustor is typically optimized for pre-mixed operation. The diffusion nozzles do not enable very good mixing of the fuel and the air to occur, resulting in the poor emissions.